A number of techniques are currently available for detection and measurement of airborne or atmospheric constituents using information from their ro-vibrational spectra. The spectral absorption lines of interest for small molecules that form such constituents are typically in the infrared region. Such techniques may be passive, in that the light originates from an incoherent source such as the sun, or active, in which light from a light source is used to illuminate a target and backscattered light is sensed by an associated detector.
The most generally used active technique is LIDAR (light detection and ranging), which involves using a laser to illuminate the target with coherent radiation for either direct or heterodyne detection of backscattered radiation. Such techniques are used commercially and are widely described in the academic literature, for example in “Laser Remote Sensing (Optical Science and Engineering), Tetsuo Fukuchi (Editor) CRC Press (28 Jun. 2005); and “Elastic Lidar”, V. A. Kovalev and W. E. Eichinger, Wiley-Interscience 2004. LIDAR systems are extensively used in atmospheric measurement, particularly by NASA. CO2 gas lasers provide acceptable levels of power in spectral ranges of interest and have been extensively used as the illumination source. Either a continuous or a pulsed laser may be used, though each has been found to have advantages and disadvantages. Continuous systems have generally not been effective for vapour phase targets, but have advantages for heterodyne detection, whereas pulsed systems have been effective for direct detection of vapour phases.
Heterodyne detection techniques involve the use of a local oscillator whose signal is combined with detected light to allow significantly greater sensitivity than is available through direct detection. In effect, beats between the local oscillator and the detected light are used to amplify the signal of interest, which can then be reconstructed by appropriate calculation. The local oscillator may be obtained in a continuous LIDAR system by splitting the light (for convenience, the term “light” will be used hereafter for all such systems, although the techniques used may be employed across a wide range of the electromagnetic spectrum) from the laser source to form two beams. While part of the light is used to illuminate the target and so provide the signal to be evaluated, another part of the light is shifted in frequency by a component such as an acousto-optical modulator (AOM) to serve as the local oscillator and subsequently combined with the backscattered signal for detection. In pulsed LIDAR heterodyne systems, this approach has not been effective and a separate local oscillator has been used which needs to be frequency stabilized to ensure frequency overlap with the backscattered radiation from the target. The pulse profile of existing pulsed laser systems can also affect temporal resolution and make relatively short range measurements difficult to achieve.
Active heterodyne detection systems using CO2 gas lasers have been used for atmospheric sensing of target molecules over significant ranges, but these systems still have significant challenges, particularly for use with gaseous targets. As can be seen from FIGS. 2a and 2b, back scattering from a solid target is much greater than back scattering from an aerosol target, because a scattering event may be over a widely distributed scattering space rather than predominantly backscattered broadly towards the source. A particularly effective LIDAR technique for detection is Differential Absorption LIDAR (DIAL), which involves taking measurements on and off resonance with the target gas species absorption and measuring the differential absorption between the two. This principle is shown in FIGS. 3a and 3b. FIG. 3a illustrates the difference in signal between on resonance and off resonance, and as is shown with FIG. 3b, the differential in received power is effective for distance measurement. Although this approach has the potential for great sensitivity, it requires very accurate control of the laser lines used. Use of CO2 gas lasers is also problematic when high sensitivity is required, as results are affected by absorption from atmospheric CO2.
US 2010/0029026 is directed to a method of constructing a mid- or far-IR device on a chip for analysing a scene. The device comprises a QCL and a QCD (Quantum Cascade Detector), preferably epitaxially grown together on the same substrate. It is suggested that the device could be constructed to allow heterodyne detection by splitting the QCL beam to use part as a local oscillator. The QCL laser and QCD detectors are constructed (using DFB techniques) each to operate at specific frequencies. To cover multiple frequencies, it is suggested to use a matrix of QCL lasers and detectors, each pair been optimised for a different frequency. In this arrangement, each laser source is carefully fixed in frequency, with pulsing techniques used to access a fixed frequency range to enable detection of a single vibration.
It is therefore desirable to produce a heterodyne detection system suitable for use to detect remote detection of target molecules over a wide range and with great sensitivity. Such a system would have particular benefits, for example in the remote detection of vapour traces from objects which it would be difficult or unsafe to inspect directly—this allows for remote inspection for gas leaks or for remote detection of explosive materials.